Motion sensors which detect movement for security and lighting applications are well known in the prior art. However, in order to make motion sensors less conspicuous, whether for aesthetic, safety or security purposes, the lens arrays utilized by motion sensors must be smaller.
Lens arrays, particularly fresnel-type lens arrays, make motion sensors more effective and economical. The individual lenslets of a lens array collect and direct infrared radiation onto a pyroelectric detector. The size and placement of the lenslets, along with other characteristics, determine the range of the motion sensor and its sensitivity to small movements. Generally, the detection range and sensitivity of the motion sensor are proportional to the size of the lens array.
Three parameters control the effectiveness of a motion sensor. Radial range is the range of the motion sensor as measured from the motion sensor to the farthest point at which it will detect motion. This parameter is limited by the collection efficiency of the lenslets and the sensitivity of the pyroelectric detector. The second parameter, angular range, is the angular spread within which the motion sensor will function. Angular range is usually limited by factors such as the curvature and size of the lens array, off-axis performance of the lenslets, and the angular acceptance of the pyroelectric detector. The third parameter is zone density which is measured as the number of detection zones or beams within an angular segment. This parameter determines the distance that a target has to move before the motion sensor detects it. Zone density is most affected by the focal length of the lenslets and the distance between the elements of a two element pyroelectric detector. Focal length is directly related to the distance from the lenslets to the pyroelectric detector.
Most motion sensors use pyroelectric detectors with two, one millimeter wide sensitive elements wherein the inner edges of the elements define a gap between the elements that is one millimeter wide. Since there is often no choice in the geometry of the pyroelectric detector used, the only way to increase the zone density has been to use longer focal length lenslets. However, this method has the disadvantage of creating a large, often unsightly and conspicuous package which is usually undesirable in most security and lighting applications.